The present invention relates to a coaxial type starter device for starting an automobile engine.
There has been known a coaxial starter device of this kind as disclosed in, for instance, Japanese Unexamined Utility Model Publication 71474/1988 (Japanese Utility Model Application 166198/1986). The conventional coaxial type starter device disclosed in this publication comprises, as shown in FIG. 11, a d.c. motor 1 having a tubular armature rotary shaft 1a. A rotary output shaft 4 which is arranged at one side in the axial direction of the d.c. motor 1, is adapted to receive a force of revolution from the armature rotary shaft 1a through a driving force transmitting device 3 including a planet gear speed reducing means 2, and is slidable in its axial direction. A pinion 5 is arranged at the front end portion of the rotary output shaft 4, and an electromagnetic switch 6 is arranged at the other side in the axial direction of the d.c. motor, which is adapted to supply a current to the d.c. motor and to push the rotary output shaft to cause the sliding movement. The armature rotary shaft 1a of the d.c. motor 1 is arranged in the same axial line as the electromagnetic switch 6.
In the coaxial type starter device having the construction described above, the rear bracket 7 of the d.c. motor 1 is provided with an opening formed in its end plane, and a power source side stationary contact 8a and a brush side stationary contact 8b project into the electromagnetic switch device 6 through the opening. On the other hand, the electromagnetic switch device 6 has a cup cylinder-shaped plunger 10 which is arranged in an exciting coil 9 so as to be slidable in its axial direction, and an end of a rod 11 arranged on the axial line of the plunger 10 is fixed to the plunger 10. The other end of the rod 11 extends in the armature rotary shaft 1a of the d.c. motor 1 through the central opening of the rear bracket 7 so that it can push the rotary output shaft 4. A pair of movable contacts 12 are supported by the rod 11 in the electromagnetic switch 6. The movable contacts 12 can be moved by the movement of the rod 11 in the axial direction by the movement of the plunger 10 so that the movable contacts 12 are brought into engagement with the stationary contacts 8a, 8b projected through the opening formed in the end plane of the rear bracket 7 so as to form a short circuit between the stationary contacts.
In the conventional coaxial type starter device, however, powder resulting from the wearing of brushes in the d.c. motor 1 may enter in the electromagnetic switch device 6 through the opening formed in the end plane of the rear bracket, or powder caused by the engagement between the movable contacts 12 and the stationary contacts 8a, 8b or resulting from the vibrations of an insulating material interposed between the movable contacts 12 and the rod 11 may fall and deposit on the inner surface, i.e., the sliding surface of a sleeve 13 fitted to the inner circumferential portion of the exciting coil 9. When such powder deposits accumulate on the sliding surface, there is a danger that the plunger can not smoothly slide. When this happens the movable and stationary contacts cannot engage, and the d.c. motor 1 becomes inoperable.